Fig. 4

Probing optical anisotropy of h-BN in the visible region. Experimental results for a 75-nm-thick and b 230-nm-thick h-BN samples. In a, the h-BN nanoflake was placed in the same orientation as in Fig. 1a; in b, the two opposite edges are not exactly parallel and there are small angles between the edges and the direction of the s-SNOM tip cantilever as shown in Supplementary Fig. 9b. There is a small frequency difference between the air modes in a and b because of the different β angles shown in Supplementary Fig. 9. Note that in b the spectrum taken at the right edge is multiplied by three